Spark plug with the insulator tip coated with a lead oxyhalide



1967 s. P. MATTARELLA SPARK PLUG WITH THE INSULATOR TIP COATED WITH A LEAD OXYHALIDE Filed April 6, 1965 )4 Kzy/W A TTORNEY United States Patent 3,345,532 SPARK PLUG WITH THE INSULATOR TIP COATED WITH A LEAD OXYHALIDE Samuel P. Mattarella, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Apr. 6, 1965, Ser. No. 445,905 Claims. (Cl. 315-143) ABSTRA CT OF THE DISCLOSURE A spark plug firing tip portion having a lead oxyhalide coating thereon which protects the glass or silica in the insulator firing tip portion from attack by low-melting lead compounds which are formed in engine operation. The lead oxyhalide coating may be bonded to the firing tip portion by reacting the lead oxyhalide compound at 1300 F. with the glass in the insulator for a time sufficient to bond the coating to the firing tip portion of the insulator.

This invention relates to spark plugs and more particularly to a spark plug insulator having an improved firing tip portion.

One of the most suitable ceramics for use as a spark plug insulator is known to be a ceramic composition of the type consisting of a glass bonded mass of corundum crystals. The corundum crystals of such a composition are enveloped by glass and are thereby bonded together into a dense centered nonporous mass wherein the large proportion of corundum being the discontinuous phase and the relatively small proportion of the glass being the continuous phase.

The principal disadvantage of such a spark plug insulator ceramic composition is the susceptibility of the insulator to attack by lead salts having a low melting temperature. Organo-lead compounds are added to most automotive fuels to improve the octane rating of the fuel. These organo-lead compounds decompose in the combustion chamber to form lead metal. Organic compounds containing halogens are added to fuels so that the halogen will react with the lead in the combustion chamber to form a volatile lead halide. These voltatile lead halide compounds are condensed during idling periods on the spark plug insulator tip where it is relatively cool. The lead halide compounds are predominantly lead oxybromide and lead oxychloride. Lead bromide, lead chloride and lead halide compounds containing materials which were present in other gasoline additives are also deposited on the insulator tip. During periods of high engine output when the spark plug insulators are hot, the lead halide salts having a low melting temperature melt and attack the insulator. It is known that the lead halide compounds attack the glass phase in the insulator rather than the corundum. It appears that the lead compounds react with the glass phase of the ceramic composition causing the glass phase to gradually corrode away. This corroding of the glass phase results in a decrease in shock resistance, mechanical strength and other desirable properties. Frequently, portions of the insulator tip will break off thereby permitting a short circuit from the shell to the center electrode. It is also believed that the glass phase, after being subjected to lead salts, becomes conductive so that a short circuit is formed along the surface of the insulator. This problem of lead attack is particularly acute in piston-type aircraft engines which are run on fuels containing 2 or 3 times the lead concentration found in automotive fuels. Moreover, piston-type aircraft engines are subjected to more severe operating conditions 3,345,532. Patented Oct. 3, i967 which tend to accelerate erosion at the firing tip portion of the insulator.

It is a basic object of this invention to provide means for inhibiting the corrosive attack of the low melting lead salts on the firing tips of spark plug insulators by providing a protective coating on the insulator firing tip which is resistant to penetration by these lead salts.

It is another object of this invention to provide a spark plug insulator having improved corrosion resistance. These and other objects are accomplished by applying a high melting lead oxyhalide coating onto the firing tip portion of a spark plug insulator and then firing the coated insulator for a sort time. The lead oxyhalide coating partially reacts during the firing with the silica glass bonding phase of the insulator and forms a coating which protects the silica from attack by low melting lead compounds Which are formed in engine operation.

Other objects and advantages of this invention will be apparent from the following detailed description, reference being made to the accompanying drawing wherein a preferred embodiment of this invention is shown.

The drawing is a side view of the improved spark plug partly in cross section and partly in elevation.

Referring now to the drawing, the spark plug 10 comprises a conventional outer metal shell 12 having a ground electrode 14 welded to the lower end thereof. Positioned within the metal shell 12 and secured in the conventional manner is an insulator 16. The insulator 16 should have a ceramic composition possessing superior electrical insulating properties at elevated temperatures, high mechanical strength, high thermal conductivity, high thermal expansion, and a high resistance to lead attack. Such a composition is disclosed in the patent to Schwartzwalder et a1. 2,760,875. Such a ceramic body may have a composition of 85.5% to 97% by weight aluminum oxide, 1.9% to 10.5% by weight silicon dioxode, 0.15% to 2.2% by weight strontium oxide and about 0.4% to 3.3% by weight magnesium oxide. This invention is not limited to an insulator having the ceramic composition given above; however, it has been found that such a composition has a higher resistance to lead attack than other ceramic compositions used for spark plug insulators. The ceramic insulator 16 is formed with a centerbore having a lower portion 18 of relatively small diameter. Positioned in the lower portion 18 of the insulator centerbore is the center electrode 20, the serrated lower end 22 thereof projecting beyond the lower tip of the insulator 16. The lower end 22 of the center electrode 20 constitutes a firing tip which forms a spark gap with the ground electrode 1 The firing tip portion 24 of the insulator 16 has an outwardly flaring counterbore in the centerbore thereby forming an inner wall portion 26 which is radially spaced from the center electrode 20. The firing tip portion 24 has an exposed inner wall 26 and an exposed outer wall 2-8 which are susceptible to corrosion attack by the lead salts that have melted. In order to inhibit this corrosion, which seriously limits the useful life of a spark plug, the inner Walls 26 and the outer Walls 28 of the firing tip portion 24 are coated with a lead oxyhalide material Stl.

The lead oxyhalide material Sil is preferably formed and applied to walls 26 and walls 28 by the following procedure. In the preferred embodiment, lead oxybromide having a ratio of 7 lead oxide molecules to 1 lead bromide molecule is prepared by weighing stoichiometric quantities of lead oxide 1562 grams) and lead bromide (367 grams). The weighed quantities of lead oxide and lead bromide are then mixed in a mortar with a pestle. After a homogeneous mixture is obtained, the mixture is removed from the mortar and placed in an alumina crucible. The alumina crucible is placed in an oven preheated to 1600 F. where it is maintained for 2 hours. The 7:1 lead 3 oxybromide reaction product material is then removed from the oven and after cooling is reground in the mortar with a pestle.

The 7:1 lead oxybromide compound is then mixed with a sufficient amount of water to make a slurry. The slurry is then applied to the lower A inch to /2 inch of the insulator firing tip walls 26 and 28 by brushing or by spraying. The coating on the firing tip portion 24 of the insulator preferably has a thickness ranging from 0.001 inch to 0.010 inch. The 7:1 lead oxybromide is reacted with the glass phase of the insulator firing tip portion 24 when the insulator assembly is fired. Normal firing time of a spark plug insulator assembly in the production line furnace ranges from to minutes at a temperature between 1300 and 1700 F. Since 7:1 lead oxybromide melts at 1365 F., this length of time is sufficient to melt a sufiicient amount of the lead compound and effect a partial reaction between the 7:1 lead oxybromide and the glass. This partial reaction at 1300 to 1700 F. in effect limits or prevents excessive corrosion of the glass by the lead 7:1 lead oxybromide. The unreacted 7:1 lead oxybromide on the walls 26 and the walls 28 act as a protective coating to prevent low melting lead oxyhalide salts from attacking the glass phase. The unreacted 7:1 lead oxybromide on the walls 26 and 28 has a sufliciently high melting point so that it will not melt when the insulator firing tip portion reaches its maximum temperature in actual engine operation. Since the 7: 1 lead oxybromide compound does not melt during engine operation, it does not attack the silica. Attack only occurs when the lead salt is in a molten state.

Essentially this invention provides for coating the insulator tip with a high melting lead oxyhalide compound which will not melt when subjected to use in the automotive engine or in an aircraft engine. This high melting oxyhalide coating forms a barrier which prevents the low melting lead halide compounds and low melting lead oxyhalide compounds from attacking the silica when these low melting lead salts are in a molten state.

It is known that the lead salt which is predominantly deposited on aircraft spark plugs is the 2:1 lead oxybromide. As shown in the following table, pure 2:1 lead oxybromide has a melting point of 1308 F. However, it should be pointed out that the 2:1 lead oxybromide which is deposited on the aircraft spark plugs is not pure due to the other additives in the gasoline. As a result, the 2:1 lead oxybromide deposited on the insulator tip has a somewhat lower point melting temperature than that given in the table below. Studies have indicated that the temperature on the spark plug insulator tip in an aircraft engine varies during operation from a temperature of 650 F. to 1370" F. Therefore, during periods when the spark plug insulator tip has a temperature over 1300 F., the 2:1 lead oxybromide salt will melt. When the 2:1 lead oxybromide melts, it will attack the silica in the insulator. However, according to this invention an aircraft spark plug insulator tip which has been coated with a preferred compound such as 6:1 or 7:1 lead oxybromide or lead oxychloride will not be attacked by the molten 2:1 lead oxybromide. The four compounds just mentioned having a 6:1 or 7:1 ratio have a melting point which is approximately 1366" F. Since this temperature is approximately the upper limit that is ever reached in an insulator tip, no corrosive attack from the preferred 6 or 7:1 lead oxyhalide would occur. Moreover, it should be pointed out at this point that there is a time factor involved in melting these lead oxyhalide compounds. Even though the melting point, for instance, of 7:1 lead oxylead bromide is 1366 F., that does not mean that the 7: 1 lead oxybrornide will melt at that temperature immediately; it may take as long as several hours at this temperature before a given quantity will melt. Therefore, in aircraft spark plugs the insulator tip should have a coating of lead oxychloride or lead oxybromide wherein the ratio of lead oxide molecules to lead halide molecules is 6:1 or 7: 1.

It has been found that in automotive spark plugs the predominant lead salts observed on the insulator tip are the 2:1 and the 1:1 lead oxybromide. In addition, there are minor quantities of the 2:1 and 1:1 lead oxychloride as well as minor quantities of lead chloride and lead 'bromide. Lead bromide, lead chloride, 1:1 lead oxybromide, and 1:1 lead oxychloride compounds have melting points below 1000 F. The temperature of the insulator tip in an automobile engine depends primarily on the speed of the engine. For example, the temperature of the insulator tip on a number of engines was determined at speeds of 70 to miles an hour and it was determined that the temperature range was from 400 to 1300 F. Thus, it can be readily seen that although the temperature in the automotive spark plug insulator tip is lower than in the aircraft spark plug insulator tip, the presence of other low melting lead halide compounds contribute to the problem for automotive use. Again, the preferred compounds for coating the insulator firing tip portion would be the 6:1 and 7:1 lead oxychloride and lead oxybromide. In addition, since the temperature of the automotive spark plugs insulator firing tip is not as high as in the aircraft spark plug, it is possible to use either the 2:1 or 3:1 lead oxychloride or lead oxybromide. These compounds would form a protective coating which would be suitable under most automotive conditions.

The following table lists high melting lead salts having a melting point in excess of 1300 F. which may be used to provide the protective coating.

Lead Oxybromide Lead Oxybromide, Lead Oxybromide. Load Oxybromide Estimated.

The preferred halogen used in the lead oxyhalide compound is bromine. Various similar lead oxyhalide compounds containing iodine, fluorine and chlorine and hav ing a melting point in excess of 1300 F. are also suitable for this invention.

While this invention has been described in terms of specific embodiments, it is to be understood that the scope of the invention is not limited thereby except as defined in the following claims.

I claim:

1. A spark plug operative at temperatures below 1300 F. comprising a metal having a ground electrode secured to one end thereof, a ceramic insulator secured within said shell and having an un-glazed firing tip portion, said firing tip portion having a thin corrosion inhibiting coating on a lead oxyhalide material bonded thereon, said lead oxyhalide material having a melting point above 1300 F.

2. A spark plug operative at temperatures below 1300 F. comprising a metal shell having a ground electrode secured to one end thereof, a ceramic insulator secured within said shell and having an unglazed firing tip portion, said firing tip portion having a thin corrosion inhibiting coating of a lead oxyhalide material, said coating having been bonded to said firing tip portion by firing at temperatures above 1300 F., said lead oxyhalide material having a melting point above 1300 F. and consisting of a compound taken from the group consisting of lead oxybromide and lead oxychloride.

3. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, a ceramic insulator secured within said shell and having an unglazed firing tip portion, said firing tip portion having a thin corrosion inhibiting coating of a lead oxyhalide material, said coating having been bonded to said firing tip portion by firing at temperatures above 1300 F., said lead oxyhalide material consisting of a compound taken from the group consisting of lead oxybromide and lead oxychloride, said lead oxyhalide material having a ratio of at least 6 lead oxide molecules to 1 lead halide molecule.

4. A spark plug as defined in claim 3 wherein said lead oxyhalide material is 7:1 lead oxybromide.

5. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, a ceramic insulator secured Within said shell having an unglazed firing tip portion and a centerbore, said insulator having an outwardly flaring counterbore in the centerbore in the firing tip portion, a thin corrosion inhibiting coating of a lead oxyhalide material covering the outer Wall of the firing tip portion of the insulator and covering the wall of the firing tip portion exposed to the centerbore, said coating having been bonded to said firing tip portion by firing at temperatures above 1300 F., said lead oxyhalide material consisting of a compound taken from the group consisting of lead oxybromide and lead oxychloride, said lead oxychloride and lead oxybromide having a ratio of at least 6 lead oxide molecules to 1 lead halide molecule.

References Cited UNITED STATES PATENTS 2,934,667 4/1960 Pincus 313-131 3,046,434 7/ 1962 Schurecht 313-431 3,278,785 10/1966 Hauth 313-l37 JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,345,532 October 3, 1967 Samuel P. Mattarella It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 13, for "sort" read short line 36, for "dloxode" read dioxide column 4, line 56, for "metal having" read metal shell having Signed and sealed this 22nd day of October 1968.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. A SPARK PLUG OPERATIVE AT TEMPERATURES BELOW 1300* F. COMPRISING A METAL HAVING A GROUND ELECTRODE SECURED TO ONE END THEREOF, A CERAMIC INSULATOR SECURED WITHIN SAID SHELL AND HAVING AN UNGLAZED FIRING TIP PORTION, SAID FIRING TIP PORTION HAVING A THIN CORROSION INHIBITING COATING ON A LEAD OXYHALIDE MATERIAL BONDED THEREON, SAID LEAD OXYHALIDE MATERIAL HAVING A MELTING POINT ABOVE 1300*F. 